Various industrial processes utilize an aqueous solution of sodium hydroxide to wash reaction products or to remove undesirable compounds as a means of purifying desired reaction products. For instance, caustic is used to wash petroleum products in order to remove undesirable compounds from a feed stock prior to polymerization of propene and butene to obtain high octane gasoline blending components. Such polymerization reactions typically take place under high pressure in the presence of a phosphoric acid catalyst and the hydrocarbon feed stock must be free of sulfur which poisons the catalyst or basic materials which neutralize the catalyst and oxygen which deleteriously affects the polymerization reaction. Accordingly, the propene and butene feed stock is washed first with caustic to remove mercaptans and subsequently with an amine solution to remove hydrogen sulfide. Subsequently, the feed stock is washed with water to remove the caustic and amine reaction product. Accordingly, the waste water stream will contain caustic, amines, and mercaptans.
In petroleum refining, chemical treatment is used to remove or change the undesirable properties associated with sulfur, nitrogen or oxygen compound contaminants in petroleum feed stocks. The chemical treatment process involves extraction or oxidation (also known as sweetening). A typical extraction process is "Merox" extraction. This process is used to remove mercaptans from propane/propylene and butane/butylene feed streams. These streams may also undergo treatment with an amine before Merox extraction to remove excess hydrogen sulfide which tends to fractionate with propane/propylene and interferes with the Merox process. A caustic prewash removes any remaining trace hydrogen sulfide prior to Merox extractions. These streams are passed up through trays of an extraction tower. A caustic solution flowing down the extraction tower absorbs mercaptan. The rich caustic is then regenerated by oxidizing the mercaptans to disulfide in the presence of aqueous Merox catalyst and the lean caustic recirculated to the extraction tower. The disulfide is insoluble in the caustic and can be separated.
Oxidation or "sweetening" is used in the purification of gasoline and distillate fractions. A common oxidation process is also a Merox process that uses a solid catalyst bed. Air and a minimum amount of alkaline caustic is injected into the hydrocarbon stream. As the hydrocarbon passes through the Merox catalyst bed, sulfur mercaptans are oxided to disulfide. The disulfide can remain with the gasoline product, since it does not possess the objectionable odor properties of mercaptans. Caustic solutions can also be used to absorb and remove hydrogen sulfide and phenol contaminants from intermediate and final product streams during the refining of petroleum. Aqueous caustic waste streams containing phenols can be recycled by reducing the pH of the aqueous caustic until the phenols become insoluble thereby allowing physical separation.
Caustic soda and soda ash have been used as an alkalinity source in the liquid scrubbing of sulfur dioxide present in gases produced from crude oil-fired steam generators. The use of caustic allows the sulfur dioxide scrubber to run at a lower pH with a higher sulfur dioxide removal capacity in comparison with the use of soda ash. Such processes result in a large amount of waste caustic solution which, heretofore, has been disposed of by sewering.
As is known, many intermediate and final streams from plants for the processing of petroleum products contain a variety of acid compounds such as hydrogen sulfide, mercaptans, phenols, thiophenols and naphthenic acids. These substances must be removed or reduced in concentration and the compounds containing sulfur must be reduced to low enough concentrations to reduce odor. Aqueous solutions of sodium hydroxide are usually used in concentrations of between five and fifteen percent by weight to treat petroleum products so as to accomplish the desired reduction in concentration of the undesired components. Spent caustic soda solutions will have different compositions depending upon whether the caustic solution has been used for purification of propane and butane gases or in the purification of petroleum feed stocks obtained from thermal and/or catalytic "cracking" or of "straight-run" hydrocarbons which are obtained by distillation of crude oil at atmospheric pressure. However, in general, the spent caustic soda solutions have pH values ranging from 12.5 to 13.5 and the following compositions expressed in percent by weight: Free caustic soda 5.0-7.5; total oils 0.5-2.0; total sulfides 0.1-3.0; cyanides 0.05-0.3; ammonia 0.05-0.4; phenols 0.2-10; lead 2.10.sup.-4 -10.10.sup.-4 ; arsenic 1.10.sup.-4 -5.10.sup.-4 ; copper 5.10.sup.-4 -50.10.sup.-4 ; cadmium 1.10.sup.-4 -5.10.sup.-4 ; and the balance being water.
In U.S. Pat. Nos. 5,589,053 and 5,667,668, an electrolysis process for removal of caustic in hemicellulose caustic is disclosed. The caustic is recovered by electrolysis in an electrolytic cell utilizing as an electrolyte a mixture of hemicellulose and caustic which is essentially free of lignin. By electrolysis, the concentration of caustic in the anolyte compartment of the cell is decreased and the concentration of caustic in the catholyte compartment of the cell is increased so as to allow recovery of about 60 to about 80% of the caustic contained in the hemicellulose caustic starting solution.
Mercaptans have been removed in the prior art from caustic scrubber solutions by blending the caustic with oxidation agents, such as tannic acid, and then blowing air through the spent caustic scrubber solutions to oxidize the mercaptans to disulfides. The disulfides are then either skimmed off as an oily layer or further oxidized to thiosulfate or sulfate using other techniques. The prior art spent caustic scrubber solutions are then either neutralized and sewered or the oxidized sulfur compounds are precipitated with precipitating agents, such as iron, and the precipitate which is formed is removed by filtration leaving the caustic solution filtrate clean enough to be returned to the scrubber. Because of the cost of the precipitation option, current prior art practice mostly employs the neutralization and sewering option. This requires large amounts of sulfuric acid which greatly increases the amount of sulfate effluent waste which is discharged from the refinery.
Regeneration of a spent caustic solution comprising oxidizing the spent caustic stream with an air/ozone gas mixture, followed by ultraviolet radiation of the oxidized spent caustic stream, is disclosed in U.S. Pat. No. 5,268,104. The treatment of spent aqueous solutions of caustic soda utilizing a combination of ozone and calcium hypochlorite is disclosed in European Patent Application 509964A1. In these references, the spent aqueous caustic solution is obtained subsequent to the scrubbing of flue gases generated, respectively, in the production of ethylene or petroleum product processing streams containing compounds such as hydrogen sulfide, mercaptans, phenols, thiophenols, and naphthenic acids.
Gaylor, U.S. Pat. No. 2,654,706, regeneration of a spent caustic solution used to scrub sour gasoline to remove oxidizable sulfur compounds such as mercaptans, hydrogen sulfides, etc. is disclosed in which the spent caustic solution is electrolyzed in a filter press electrolysis cell having a diaphragm and insoluble electrodes. Following electrolysis, the disulfides produced are separated prior to passing the regenerated caustic back to the scrubbing process. The examples in Gaylor show the use of electrolytic cell current densities of generally less than 0.2 amps per square inch and cell voltages of about 6 volts or the minimum current density required to oxidize the sulfur containing impurities in the spent caustic solution. Just enough oxygen is generated by electrolysis of the spent caustic solution to oxidize all of the mercaptans present to disulfides. The disulfides are physically separated outside the electrolysis cell in a separation vessel. It is apparent that the process of Gaylor is intended to replace the prior art use of oxidation agents, such as tannic acid, and blowing air into the spent caustic solution to oxidize the mercaptans to disulfides. The Gaylor process would be a more expensive process than the air oxidation step of the prior art but, more importantly, the Gaylor process omits mention of the fact that during electrolysis, some of the sulfur compounds will be oxidized to thiosulfates or sulfate. These oxidized sulfur compounds will eventually build up in the spent caustic electrolyte to the saturation point so as to require either a sulfate removal step or removal by the use of vacuum crystallization or the neutralizing of the spent caustic solution followed by disposal to the environment of the oxidized spent caustic solution. As indicated above, in the Gaylor process the disulfides produced during electrolysis of the spent caustic solution are removed to the environment without further oxidation. Under present or contemplated governmental restrictions, disposal of these disulfides may represent an environmental liability, thus requiring destruction prior to discharge to the environment.
In Rippie et al, U.S. Pat. No. 2,859,177, a spent caustic solution used in the scrubbing of sour gasoline, in order to remove oxidizable sulfur compounds, is regenerated by reaction, outside an electrolysis cell with oxygen generated by electrolysis. The regenerated caustic solution subsequent to regeneration is passed to a disulfide scrubber prior to recycling the regenerated caustic back to the scrubbing stage of the process. As in Gaylor, Rippie et al discharges disulfides to the environment or recovers the disulfides for sale. Currently, disulfides have little or no chemical value and, accordingly, would be required to be discharged to the environment subsequent to recovery after scrubbing in the process of Rippie et al. In addition, the build-up of oxidized thiosulfate or sulfate compounds in the regenerated caustic solution would require, as in the case of Gaylor, either a sulfate removal step, such as vacuum crystallization, or chemically neutralizing the regenerated caustic solution prior to disposal to the environment.
In one embodiment of the process of the present invention by the use of an electrolysis cell operating at about 5 to about 10 times the current density and at about half the voltage utilized by Gaylor, a caustic can be regenerated and mercaptans and disulfides can be converted to elemental sulfur, which can be recovered for chemical value by filtration, and/or an aqueous solution comprising sulfates, which are water soluble and can be disposed of without liability to the environment. In the electrochemical cell utilized in the process of the invention, excess oxygen is liberated in the anode compartment of the cell which results in the conversion of mercaptans, disulfides, and sulfides present as impurities in the spent caustic solution to elemental sulfur and/or sulfates. Rather than vent the excess oxygen to the environment during processing, the excess oxygen can be passed to a column for pre-treatment of the feed spent caustic solution. The use of a cationic permselective membrane as a cell separator in the electrolysis cell utilized in the process of the invention allows the formation in the cathode compartment of the cell of a pure sodium hydroxide upon the feeding of deionized water to the cathode compartment. A porous membrane cell separator can be used where recovery of a pure sodium hydroxide solution is not required. Accordingly, by use of the process of the invention, it is possible to reduce the purchase of caustic in an amount equivalent to the amount of the sulfur compounds scrubbed from the hydrocarbon process streams and discharged as sulfates. The process of the invention is applicable in one embodiment to destroy or convert to a benign, environmentally non-toxic state the sulfur containing compounds or other organic ingredients removed by caustic scrubbing of a hydrocarbon process stream such as those streams from olefin plant, ethane and propane crackers as well as sour gasoline process streams.